1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, a facsimile apparatus or the like, which transfers an image on an image bearing member onto a recording material carried on a recording-material carrying member.
2. Description of the Related Art
Image forming apparatuses have been produced in which image formation is performed by conveying a recording material, such as paper or the like, in a state of being attracted on an endless belt (a recording-material carrying member), and transferring a visual image formed on an image bearing member onto the recording material at a transfer portion.
An example of such image forming apparatuses will be briefly described with reference to FIG. 7.
In FIG. 7, within a main body of an image forming apparatus 100, a photosensitive drum 105 is disposed at a central portion. A pre-exposure lamp 101, a photosensitive drum charger 102, an exposure apparatus 103, a developing unit 104, a transfer charger 107 and a cleaner 108 are provided around an outer circumference of the photosensitive drum 105.
A recording-material carrying member (hereinafter termed a "transfer belt") 106 is provided near the photosensitive drum 105. A toner image formed on the photosensitive drum 105 is transferred onto a recording material P carried and conveyed on the transfer belt 106.
The toner image on the recording material P is fixed by a fixing unit 109 by means of heat and pressure, and the recording material P having the toner image transferred thereto is discharged to the outside of the apparatus as a recorded image.
In the exposure apparatus 103, a laser beam emitted from a light source (not shown) provided at an upper portion of the main body of the apparatus is converted into a scanning beam by a rotating polygonal mirror. The scanning beam is reflected by a reflecting mirror and is condensed onto the generatrix of the photosensitive drum 105 by an f.theta. lens to expose the photosensitive drum 105. Thus, a latent image corresponding to an image signal is formed on the photosensitive drum 105.
A predetermined amount of toner is filled in the developing unit 104 from a toner supply device (not shown). The latent image on the photosensitive drum 105 is developed by the toner, to provide a visualized toner image.
The recording material P is supplied from a recording-material cassette (not shown) or a manual-insertion sheet feeding tray 117 to the transfer belt 106 via a plurality of conveying rollers and a pair of registration rollers 113, and is further fed to a transfer portion facing the photosensitive drum 105 by being conveyed by the transfer belt 106.
The transfer belt 106 is made of a material obtained by dispersing a conductive filler, such as carbon black or the like, in a sheet of a dielectric resin, such as a polycarbonate resin, a polyethylene terephthalate (PET) resin, a polyvinylidene fluoride resin, a polyurethane resin, a polyamide resin, a polyimide resin or the like, in order to reduce the volume resistivity of the insulating resin to about 10.sup.7 -10.sup.11 .OMEGA..multidot.cm, and has an endless shape by superposing and bonding both end portions of the sheet, or the shape of a seamless belt.
When the transfer belt 106 is rotated by a driving roller 111 in conjunction with an idler roller 112 and reaches a constant speed, the recording material P is fed from the pair of registration rollers 113 to the transfer belt 106, and is conveyed to the transfer portion. At the same time, an image write signal is generated, and image formation is performed on the photosensitive drum 105 at a certain timing based on the signal.
By providing an electric field or electric charges from the transfer charger 107 at the transfer portion below the photosensitive drum 105, the toner image formed on the photosensitive drum 105 is transferred onto the recording material P. When the recording material P is fed from the pair of registration rollers 113 onto the transfer belt 106, the recording material P is immediately grasped by an attracting charger 114 and a roller 115 facing it, together with the transfer belt 106. By being supplied with an electric field of electric charges from the attraction charger 114, the recording material P is electrostatically held and conveyed on the transfer belt 106.
A noncontact-type charger utilizing corona discharge, or a contact-type charger using a charging member, such as a charging roller, a charging brush or a charging blade, is used as the transfer charger 107. Noncontact-type chargers have problems such as, generation of ozone, weakness against variations in the temperature and humidity of atmospheric air because charging is performed via air, resulting in, for example, unstable image formation. On the other hand, contact-type chargers have advantages, such as no generation of ozone, strength against variations in the temperature and humidity of atmospheric air, and the like.
Charges on the recording material P having the toner image transferred thereto are removed by a separation charger 116 at a downstream portion in the conveying direction of the transfer belt 106. The electrostatic attracting force of the recording material P is thereby attenuated, so that the recording material P is separated from an end portion of the transfer belt 106.
Particularly, since in a low-humidity environment, the recording material P is dried and has a high electrical resistivity, the electrostatic attracting force with the transfer belt 106 is large. Hence, the effect of the separation charger 116 is large. Usually, the separation charger 116 removes charges from the recording material P in a state in which the toner image is unfixed. Accordingly, a noncontact-type charger is used as the separation charger 116.
An AC voltage of about Vp-p=10 kV, and a frequency of 500 Hz is used as the output of the separation charger 116. In order to prevent failures in the obtained image, such as dispersion of toner particles, and the like, a positive or negative DC component of about +100 .mu.A is, in some cases, superposed on the AC output.
The separated recording material P is conveyed to the fixing unit 109. The fixing unit 109 includes a fixing roller, a pressing roller, a heat-resistant cleaning member for cleaning these rollers, heaters disposed within the respective rollers, a coating roller for coating a release oil, such as a dimethylsilicone oil or the like, on the fixing roller, a reservoir for the oil, and a thermistor for controlling the fixing temperature by detecting the surface temperature of the pressing roller.
The toner image formed on the recording material P is fixed to provide a copy image, and the recording material P having the fixed toner image is discharged onto a discharged-sheet tray.
Toner particles remaining on the photosensitive drum 105 after the image transfer are cleaned and removed by the cleaner 108, and the photosensitive drum 105 is used for the subsequent latent-image formation.
Toner particles and other foreign matter remaining on the transfer belt 106 and accumulated charges after separating the recording material P are cleaned by being passed between a conductive fur brush 110 and a grounded driving roller 111 facing it. An idler roller 112 provides tension on the transfer belt 106. A conductive web (nonwoven fabric) may be used instead of the conductive fur brush.
The above-described image forming apparatus uses a pair of rollers, i.e., the attraction charger 114 and the roller 115 facing it, as electrostatic attraction means for the recording material P. However, as in the case of the transfer charging means, a noncontact-type charger utilizing corona discharge, or a contact-type charger using a charging member, such as a charging roller, a charging brush or a charging blade, may, of course, also be used as the electrostatic attraction means.
However, when a contact-type charger is used, since tight contact between the recording material P and the transfer belt 106 is not guaranteed, it is necessary to secure tight contact between the recording material P and the transfer belt 106 by separately providing a pressing member.
In order to realize stable electrostatic attraction and conveyance of the recording material P, it is, of course, necessary to perform attraction charging for the entire width of the recording material P (a length of the recording material P in a direction substantially orthogonal to the conveying direction of the recording material P).
Japanese Patent Laid-Open Application (Kokai) No. 2-157779 (1990) discloses that projections and recesses of a carried sheet constituting a transfer drum are corrected by correction means at the upstream side in the direction of rotation of the carried sheet of an attraction corona charger for electrostatically attracting a transfer material onto the carried sheet. There is also disclosed that the conductive roller has the shape of a crown. This structure is adopted for the purpose of providing a uniform contact pressure in the thrust direction by preventing a decrease in the contact pressure at a central portion compared with the contact pressure at both end portions due to deformation of the conductive roller in the thrust direction. There is also disclosed that a pair of rollers for correcting projections and recesses of the carried sheet before electrostatically attracting the transfer material onto the carried sheet are provided, and that at least of one of the pair of rollers may be formed in the shape of a crown, for the above-described reason.
In the image forming apparatus shown in FIG. 7, however, for example, when a recording material left in a high-humidity or low-humidity environment is used, or recording is performed on a second surface of a recording material in duplex printing in which images are formed on both surfaces of the recording material, the image quality is, in some cases, greatly degraded.
This is because the recording material is deformed, and a portion having an inferior contact state with the transfer belt 106 is generated due to degradation in the flatness of the recording material.
For example, paper, serving as a recording material, mounted within a sheet feeding cassette absorbs or discharges water depending on the temperature and humidity of external air. The paper is deformed substantially as a part of a spherical surface in this process.
When such paper is fed onto the transfer belt 106 and passes through the pair of rollers, i.e., the attraction charger 114 and the roller 115 facing it, the paper is squeezed by the pair of rollers and is adjusted with the transfer belt 106 having high flatness. As a result, the deformation of the paper is accumulated as the paper passes, and the paper is, in some cases, held in a state in which a central portion in the thrust direction of a trailing-edge portion of the paper is separated from the transfer belt 106. Since this separated portion produces a gap at the transfer portion, the image on the photosensitive drum 105 cannot be excellently transferred onto the paper, thereby producing a failure in the obtained image.
Similarly, in an image forming apparatus according to the above-described patent application, also, a failure in image transfer, in some cases, occurs because the transfer material is not excellently attracted onto the carried sheet, thereby causing prominent degradation in the image quality. It can be considered that this is for the following reason. Namely, since the carried sheet and the transfer material are not tightly pressed between the conductive roller, serving as an electrostatic attraction member, and the attracting corona charger, the transfer material is not sufficiently attracted onto the carried sheet in a close contact state, when a transfer material left in a high-humidity or low-humidity environment, or a transfer material on one surface of which a toner image has been transferred and fixed in duplex printing is used. It has also become clear that, since there is a considerable distance between a pressing position where the transfer material is pressed against the carried sheet by the pair of rollers and an electrostatically attracting portion where the transfer material is electrostatically attracted onto the carried sheet by the attraction corona charger and the conductive roller, inferior contact between the transfer material and the carried sheet occurs, thereby producing a gap between the transfer material and the carried sheet or crease in the transfer material, resulting in a failure in image transfer.